Laser diodes are used in many fields of information transmission. Laser diodes are well suited for modulating the generated laser beam with respect to its intensity. As a result of their small size and their easily implemented electronic controlling, laser diodes can attain very high data transmission rates. This is especially advantageous both in the transmission of messages as well as in the recording and reproducing of information.
One problem exists with laser diodes, however, in their characteristic of spontaneously jumping between different modes within certain brightness ranges. This effect is known as mode hopping. If laser diodes are being used for the scanning or recording of information on a data carrier such as photographic film, mode hops can occur with the laser diodes as a result of optical reflexes that arise from surfaces in the beam path which have not been perfectly dereflected, or at the surface of the film that is to be exposed. In combination with the partially reflective light emission surface of the laser diode, this reflection location forms an external resonator whose resonant frequencies (i.e. Fabry-Perot modes) are determined by the propagation time of the light wave from the laser mirror to the reflection location and back. For an impact point that is at a distance L.sub.ext, resonances with a frequency interval (i.e. mode interval) of df.sub.ext =c/(2.multidot.L.sub.ext), where c=3.multidot.10.sup.10 cm/s which describes the speed of light.
With an external resonator of 30 cm, for example, the interval of the external resonant frequencies is 500 MHz. In this case, the oscillation frequency of a laser diode would hop back and forth unpredictably among several of these resonant frequencies, with each mode hop linked with an abrupt change in the laser power.
Several solutions have already been suggested in order to prevent these mode hops. By way of example, U.S. Pat. No. 4,817,098 discloses a regulating system in which the laser power is kept constant by means of various measures. One way is to measure the temperature of the laser diode in order to avoid hops. Another way is to keep the laser diode temperature fluctuations as small as possible by means of temperature control, and thus achieve a light output of the laser diode that is as uniform as possible. U.S. Pat. No. 5,283,793 teaches a high-frequency signal to be superimposed on the image signal to prevent mode hops. In addition, U.S. Pat. No. 4,799,069 and U.S. Pat. No. 5,386,124 discloses that the laser diodes be switched off briefly between two image signals in order to avoid mode hops.
From the state of the art described above, it is known that the occurrence of mode hops can be suppressed by means of a high-frequency modulation of the laser diode current with a periodic signal. As a result of the periodic modulation, the spectrum of the modulated laser light exhibits a number of discrete side lines at the interval of the modulation frequency.
Experimental studies have now shown that effective suppression of mode hops with periodic modulation occurs only if the frequency interval of the modulation side lines exactly corresponds to the frequency interval of the external resonator modes, or at least amounts to an integer fraction of this mode interval. In accordance with that, in the case of periodic modulation the modulation frequency, f.sub.M must be exactly matched to the frequency interval df.sub.ext or an integer fraction of the same (i.e. f.sub.M =df.sub.ext /n). This matching is technically difficult, and in addition, it differs from device to device. A serious disadvantage of the periodic modulation technique is due to the fact that an effective suppression of the mode hops which occur is only possible if several reflection locations are present in the beam path of a device at varying distances to the laser diode.